Support strictNullChecks in intrinsics.ts
Support strictNullChecks in intrinsics.ts

Previously, GetIntrinsic returned a type that could be `undefined`.
That's fixed in this commit. Note that intrinsics.ts may be removed
completely in #105; if that PR lands first then we'll remove this
commit.

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill